Electroly recovery of chlorine and hydrogen from aqueous hydrochloric

ABSTRACT

AQUEOUS HYDROCHLORIC ACID IS ELECTROLYZED IN A DIAPHRAGM CELL TO RECOVER CHLORINE AND HYDROGEN THEREFROM. THE ACID TO BE ELCTROLYZED SHOULD HAVE A CONCENTRATION OF FROM 18 TO 28% BY WEIGHT OF HCL AND SHOULD BE DEGRADED BY NOT MORE THAN 2% BY WEIGHT DURING ITS PASSAGE THROUGH THE CELL. THE MEAN TEMPERATURE IN THE CELL IS KEPT AT FROM 75* TO 90*C.

United States Patent O ifice 3,756,930 Patented Sept. 4, 1973 U.S. Cl.204-429 3 Claims ABSTRACT OF THE DISCLOSURE Aqueous hydrochloric acid iselectrolyzed in a di aphragm cell to recover chlorine and hydrogentherefrom. The acid to be electrolyzed should have a concentration offrom 18 to 28% by weight of HCl and should be degraded by not more than2% by weight during its passage through the cell. The mean temperaturein the cell is kept at from 75 to 90 C.

In the chlorinating substitution of organic compounds, half of thechlorine originally used is obtained as hydrogen chloride. Since thiscannot always be used and nowadays disposal thereof with efiluents isoften out of the question, the need for recovery of the chlorine fromhydrogen chloride gains continually in importance. Two methods have beenadopted: hydrogen chloride is oxidized with air or oxygen in thepresence of catalysts, or hydrochloric acid is first formed from thegaseous hydrogen chloride by absorption and the resultant acid issubjected to electrolytic decomposition to recover chlorine andhydrogen. These methods of recovering chlorine differ from the widelyused chlor-alkali method in that it is possible to produce chlorinewithout the simultaneous production of alkali and therefore thealkali-chlorine balance is not affected.

In the electrolytic recovery of chlorine from aqueous hydrochloric acid,which is preferably carried out in diaphragm cells, the energyconsumption plays a significant part as in all electrochemicalprocesses. There has therefore been no lack of attempts in the course ofthe years to decrease the consumption of energy as far as possible bydecreasing the achievable cell potential.

This object may be achieved for example by appropriate choice of thediaphragm which prevents mixing of the gases formed on the cathode andanode sides but does not impede the flow of current. Another possibilityis to decrease the resistance in the electrolyte by carrying out theelectrolysis in the region of maximum conductivity of hydrochloric acid,or to decrease the hydrogen overvltage at the graphite cathodesgenerally used. The hydrogen overvoltage may be lowered for example inknown manner by adding certain metals to the electrolyte, the addedmetal cations being deposited in the course of the electrolysis on thecathodes and the electrolytic decomposition takes place at the layers ofthe electrodes thus activated with a decrease in the overvoltage. Metalsof the platinum group, copper, nickel, antimony, silver, molybdenum andcobalt are preferred as metals and they may be added to the electrolyteseither batchwise or continuously.

In carrying out the hydrochloric acid electrolysis, acids having an HClconcentration of more than 20% up to about 26% by weight are used. Therate of flow of the acid is controlled so that a concentration gradientbetween the acid introduced into the electrolyzer and the acid withdrawnof more than 3% by weight, for example of 5% by weight, is maintained.Since there is no freedoin as regards the temperature of the acidleaving the cell because of its HCl partial pressure (generally atemperature of the effluent acid of C. should not be exceeded) it isnecessary to cool the acid strongly before it enters the cell. In thisway the mean temperature in the cell is kept fairly low. The temperaturerange used is therefore one in which the hydrochloric acid has a lowconductivity 30 that an increased amount of energy has to be used.

We have now found that the electrolysis of hydrochloric acid can becarried out with economically favorable current consumption and lowpotential and without adding substances to decrease the overvoltage whenthe acid to be electrolyzed has a concentration of from 18 to 28% byweight of HCl, the acid electrolyzed is degraded to the extent of notmore than 2% by weight of HCl and the mean temperature of the acid inthe cell is kept at from 75 to C.

The acid being electrolyzed is preferably degraded by from 0.5 to 1.6%by weight.

The low degradation of the hydrochloric acid is achieved by passing theacid to be electrolyzed at an increased rate of flow through the cathodeand anode chambers at the known and conventional current densities offrom 2000 to 5000 amperes per square meter. In this way it is possibleto eliminate the resistance produced by the gas bubbles forming at theelectrodes, as the bubbles are rapidly detached from the cathode andanode and entrained. A considerable decrease in the cell potential isthus achieved by simple means.

Another advantage of the method in accordance with the invention is thatit is possible to use a higher mean cell temperature. Because of theincreased throughput of acid it is possible to allow the acid to enterat a relatively high temperature and a further gain of potential isachieved because of the higher conductivity of hydrochloric acid atincreasing temperatures.

The following example illustrates the invention.

EXAMPLE (A) A 25% by weight hydrochloric acid is passed through adiaphragm cell (having an anode area of 2.32 m. and consisting of thirtysuccessive individual cells) at such a rate that the concentration ofthe degraded acid leaving the cell is 21.5%. At a current of 9400amperes a cell potentail of 2.25 volts is measured. The mean temperatureof the acid is 65 C.

(B) If the acid being electrolyzed is degraded from 25% by weight toonly 23.5% by weight, the cell potential is 2.03 volts at a meantemperature of the acid in the cell of 77 C.

We claim:

1. In a process for the electrolytic recovery of chlorine and hydrogenby continuous flow of aqueous hydrochloric acid through a diaphragm cellhaving anode and cathode chambers maintained at a current density offrom 2000 to 5000 amperes per square meter, the improvement whichcomprises introducing into said cell said aqueous 5 hydrochloric acid tobe electrolyzed at a concentration of 18 to 28% by weight of HCl,conducting said acid through said cell at a fiow rate which issufficiently high to degrade theacid by not more than 2% by weight andmaintaaining the mean temperature of the acid in the cell at from 75 C.to 90 C.

2. A process as claimed in claim 1 wherein said flow rate of the acid ismaintained sufiiciently high so as to degrade the acid by from 0.5 to1.6% by weight.

3. A process as claimed in claim 1 wherein said aqueous hydrochloricacid being electrolyzed is substantially free of metal additives whichlower the hydrogen overvoltage at the cathodes of the cell.

4 References Cited UNITED STATES PATENTS 2/1930 Low 204-128 2/1966Messner 204-128 Us; Cl. X.R.

Da swimmer a 197? Patent NO "3 1175 6 @930 fi) Gueriith ex Weiss, HeinzWeller and. Ewalci p tified that erfor appears in the above-idehtifiedpatent It is ear at are hereby corrected as shown below:

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